In the photographic field, there is a continuous demands to obtain silver halide photographic emulsions having lower fog and better speed and stability under storage condition.
It is well known in the art that speed and fog of a silver halide photographic emulsion are dependent upon each other and it is difficult to increase speed without a corresponding increase in fog and vice versa.
Sensitivity of a silver halide photographic emulsion can be improved by (1) increasing the number of photons absorbed by each silver halide grain, (2) increasing the conversion efficiency of photoelectrons generated by light absorption into silver clusters, or (3) improving the development activity to have a complete development of silver clusters.
A practical way to increase the number of photons absorbed by each silver halide grain is by increasing the grain size and having a high concentration of sensitizing dye absorbed on the surface of the silver halide grains. However, this method reduces the image quality. Moreover, high concentration of sensitizing dyes can have an adverse effect on the sensitivity of the silver halide grains due to the tendency for recombination of trapped photoelectrons with free dye photoholes and destruction of the latent image (silver clusters).
The improvement of developing activity has the drawback of adversely affecting graininess of the resulting image.
Increasing the conversion efficiency of photoelectrons generated by light absorption into silver clusters is one means of reducing the effects which decrease the probability of recombination of photoelectrons and positive holes generated by light absorption.
It is known in the art that reduction sensitization methods in which a reducing agent is introduced into the vessel during silver halide preparation can effectively reduce the recombination by formation of fine silver cluster (Ag.sub.2) having no development activity in the interior of the silver halide grains or on the surface thereof. It is also known in the art to form molecular clusters of Ag.sub.2 S (very effective photohole acceptors) by introducing sulfur compounds during silver halide preparation.
Examples of these techniques can be found in EP 435,270; EP 371,338; U.S. Pat. No. 5,254,456; U.S. Pat. No. 5,079,138; U.S. Pat. No. 5,368,999; EP 438,791; EP 434,012; U.S. Pat. No. 5,290,673; U.S. Pat. No. 5,061,614; EP 552,650; and EP 407,576.
EP 435,270 and EP 434,012 disclose the use of oxidizing agents during silver halide emulsion making (in particular, the use of thiosulfonate derivatives).
EP 371,338 and U.S. Pat. No. 5,061,614 disclose a silver halide emulsion reduction sensitized with a reducing agent in the presence of a thiosulfonic acid derivative during precipitation of silver halide grains.
U.S. Pat. No. 5,254,456 and U.S. Pat. No. 5,079,138 disclose a silver halide emulsion reduction sensitized with ascorbic acid or derivatives thereof (in an amount of from 10.sup.-1 to 10.sup.-5 mol/molAg) in the presence of a thiosulfonic acid derivative during precipitation of silver halide grains.
U.S. Pat. No. 5,368,999 discloses a silver halide emulsion reduction sensitized during precipitation of silver halide grains and then added with a thiosulfonic acid derivative.
EP 438,791 discloses the use of a thiosulfonic acid derivative during manufacturing of a silver halide tabular emulsion containing not less than 3% of silver iodide.
U.S. Pat. No. 5,290,673 discloses a silver halide emulsion reduction sensitized with ascorbic acid or derivatives thereof (in an amount of from 10.sup.-1 to 10.sup.-5 mol/molAg) during precipitation of silver halide grains and further added with a mercaptotetrazole derivative.
EP 552,650 discloses a reduction sensitized silver halide emulsion further containing a polyvalent metal (such as Fe, Ir, Cd, Pb, In, Os, and Re) in an amount of not less than 10.sup.-6 mol/molAg.
EP 407,576 discloses a silver halide emulsion reduction sensitized in the presence of an oxidizing agent, said silver halide emulsion being prepared by adding fine silver halide grains into a reaction vessel for causing nucleation and/or growing of silver halide grains.
However, the above mentioned technique still requires an improvement for the prevention of the surface molecular cluster aggregation and formation of polyatomic Agn and polymolecular (Ag.sub.2 S)n (wherein n is higher than 2), which decrease speed and increase fog.
U.S. Pat. No. 5,114,838 discloses the use of azaindene derivatives in a manufacturing process of a silver halide emulsion subjected to reduction sensitization, preferably in the presence of a thiosulfonic acid derivative. However, the azaindene derivative is added after reduction sensitization and together with or before chemical sensitization.
U.S. Pat. No. 4,610,958 discloses the use of a tetraazaindene derivative in the process of manufacturing monodispersed octahedral or tetradecahedral silver bromoiodide emulsions.
U.S. Pat. No. 4,078,937 discloses the use of tetraazaindene derivatives in the process of manufacturing a sulfur sensitized ammoniacal silver halide emulsion having grain size not exceeding 0.5 .mu.m.